This invention relates to the generation of cosine waves from sine waves, and more particularly to a frequency insensitive sine wave-to-cosine wave converter having a substantially instantaneous response characteristic.
In a phase controlled converter or cycloconverter, a phase control technique that employs cosine timing waves is commonly used to linearize the transfer function between the input control signal and the ideal output voltage. In a three phase system in which the supply voltages are balanced in amplitude and phase, a frequency insensitive cosine timing wave can easily be generated by addition and subtraction of phasors. In a single phase system, the cosine wave is normally generated by integrating a sine wave, however a difficulty arises when the sine waves have a variable frequency since the amplitude of the cosine waves is then inversely proportional to frequency. While the prior art teaches the multiplying of the amplitude of the cosine wave by a voltage proportional to the frequency, the frequency-to-voltage converter introduces harmonics that are normally eliminated by a filter. This scheme is satisfactory in steady state operation but if the frequency drifts in a transient manner the asymmetrical time lags in the integrator and frequency-to-voltage converter causes transient drift in the cosine wave amplitude. The result of using a variable amplitude cosine timing wave in the control of a converter system is that the firing angle of the SCR's or other devices drifts and this in turn causes drift of the output voltage. When the phase controlled converter operating frequency changes to a high value, the control circuit depending upon operation as a rectifier or inverter will lock up in the advance limit and retard limit modes respectively, and a cycloconverter will generate flat topped waves thereby introducing excessive harmonics into the load and source. Frequency insensitive cosine timing waves eliminate these problems and maintain a truly linear transfer characteristic, and additionally the response of the converter is almost instantaneous for any transient frequency deviation.
Although suitable for many applications, the sine wave-to-cosine wave converter herein described has particular utility as a cosine timing wave generator used in the control of a cascaded high frequency link cycloconverter or cycloinverter system such as is disclosed in U.S. Pat. No. 3,742,336 to B. D. Bedford and in U.S. Pat. No. 3,882,369 to William McMurray, both assigned to the assignee of this invention. In this converter system, as typically used with an inductive load, variable commutation energy is provided by operating the parallel resonant circuit in the link above its resonant frequency. The successful operation of this system under steady state and transient conditions needs a cosine wave converter with an instantaneous response which generates cosine timing waves whose magnitude faithfully tracks the input signal magnitude regardless of frequency.